Electronic devices continue to become smaller, driven by requirements of portable devices and by the requirement to have smaller interconnections between devices so as to increase their speed. As a result, the distance between the pad connections of the device such as a semiconductor chip, and the wiring circuitry or printed circuit contact locations, where the device is attached to a wiring substrate, such as a printed circuit card, hereinafter the card, through conductive joints, becomes smaller and any expansion mismatch between the materials involved becomes more difficult to handle. The present downsizing trend is toward contact separations on the card that are in the sub millimeter range. The conductive joints themselves are usually small volume elements of solder or solderable metal that have been subjected to a temperature and duration processing operation known in the art as a processing window which produces reflowing into a joint between the pad and another part of the package such as the contact on the card.
The contacts on the card are usually of circular shaped metal, typically copper, to which printed conductors are attached that go to other electronic devices, interconnections or power distribution locations or planes. It is also common practice to place vias, or conducting holes, at the locations of some or all of the contacts. In this way the interconnections to the contacts may be run on internal layers of a multilayer wiring support member or card. Alternatively, contact may be made directly, to internal voltage or ground planes, of a multilayer printed circuit card.
When contact pitches approach the submillimeter range, it becomes difficult at the present state of the art to create vias in a printed circuit card for each contact and so wiring from one device to another device may be accomplished in these situations by also running fine lines between contacts, on the top layer or mounting surface of the card. As the size of the interconnections and the wiring joining them become smaller and enter the submillimeter range the effect from damage from expansion mismatch of the materials in the parts of the assembly becomes a more pressing consideration. The size of the electronic device and the number of contacts that can be reliably wired, then becomes a function of how many lines can be placed in the space between contacts. Present printed wiring techniques are in the direction of only allowing roughly a 75 micrometer line width and a 100 micrometer space to another line or surface mount contact. Such spacing imposes a severe constraint on the reliable wireability and long term reliability in service of fine pitched electronic devices.
Progress has been made in the art as described in U.S. Pat. No. 5,859,474 wherein the use of elongated or elliptical shaped pads aligned in a certain direction permits wider spacing between pads in the direction of alignment and more interwiring in those spaces.
To form the conductive joints that join the pads on the electronic devices to the contacts on the circuit card it is the standard practice to position small volume quantities of a solder paste, or a slurry of fine solder particles in a fluid, such as through a screen or a stencil onto the contacts of the printed circuit card, forming a small volume element which is then fused to the contact. The electronic device is then placed precisely with the pads on the screened fused elements on the card contacts and heated in a single or in multiple reflow operations at solder reflow temperatures where conductive joints are formed between the contacts on the card and the pads on the electronic device.
Heretofore in the art there have been two major limitations in the formation of conductive joints when the size of the contacts approach one millimeter or less. The first is that when the dimension of the contact becomes as small as one millimeter, the conductive paste in a screen stencil hole may not always separate from the bulk of the paste as it is screened, or wiped, across the depostion hole, resulting in situations where the paste is retained in the hole in the screen when the screen is lifted. The second is that, a difference in thermal coefficent of expansion of the parts involved such as between that of the device and that of the circuit card operates to produce differential motion which produces a resulting plastic deformation the effect of which on such contacts as the solder columns always being similarly concentrated so that early fatigue and ultimately joint failure occurs.
The objects of this invention are to increase wiring flexibility by increasing the number of wiring lines that can be placed between two adjacent contacts in an array where the contacts are 1 mm or less for connection in the interface between a pad on a surface mount electronic device and a contact on a circuit card; to improve the formation of conductive joint members in the depositon of conductive paste onto the contacts on a circuit card; and, to improve the reliability of those conductive joints under the stress conditions induced by thermal cycling in manufacturing and in service.